The invention relates to a piezoelectric audio transducer comprising a piezoelectric vibrator which is formed by bonding a piezoelectric ceramic element to a metallic diaphragm; the vibrator is supported by a casing having an acoustic effect and the like.
The piezoelectric buzzer has an advantage in that it is not only small-sized, thin, light in weight, power saving and durable but is also capable of producing a variety of tones in combination with various drive circuits. Thus, the piezoelectric buzzer has come to replace the conventional electromagnetic buzzer as an alarm in various fields.
The piezoelectric buzzer comprises two types: an audio transducer in which a two-terminal vibrator is used, as shown in FIGS. 1 and 2, and an audio transducer in which a three-terminal vibrator is used, as shown in FIGS. 3 and 4. With both types, the wider the range of their use becomes, the greater is the demand for a better sound, that is, for a lower frequency.
In order to meet the demand for a lower frequency and to simply the supporting method, the casing supporting the outer periphery of the diaphragm was conventionally secured to the mounting body by means of screws or the like, as shown in FIGS. 5 and 6.
In the case of this supporting system in which the outer periphery of the diaphragm is completely secured to the mounting body, the casing supporting the diaphragm is completely independent of the vibration, of the vibrator whereby it is difficult to get the vibrator to vibrate.
Compared with the audio transducer comprising a two-terminal vibrator, the audio transducer in which a three-terminal vibrator is used creates many problems to be solved.
Theoretically, the audio transducer comprising a three-terminal vibrator has an advantage in that the cost of the drive circuits is drastically reduced since the feedback electrode enables the simplification of the drive circuits by using a self-excited vibration system. Conventionally, however, said system has been considered to be practically impossible in the case of the construction in which the diaphragm is securely supported by its outer periphery when connected to the self-excited vibration circuit.
The reason why stable vibration is not obtainable may be attributable to an insufficient amount of feedback signal.
The self-excited vibration circuit requires a feedback signal which is sufficiently large to switch a drive transistor on and off.
To be more precise, when the outer periphery of the diaphragm is securely supported, the resonant impedance of the audio transducer is far smaller than that when the node of free vibration is supported.
Thus, the three-terminal audio transducer in which the node of free vibration is supported vibrates with stability, whereas the three-terminal audio transducer in which the outer periphery of the diaphragm is supported vibrates with less stability.